The present invention relates to a method for detecting faults in the operation of sensors and more particularly relates to such a method for detection faults in sensors with at least two decoupled signal-transmitting elements with contacts and with a closed conductor loop, in which a respective signal is applied to the input of each signal-transmitting element via an associated test output, which signal is supplied to an associated input channel at the output end.
In processes in safety engineering, for example in applications such as emergency-stop buttons or door switches for protective door interlocking, industrial robots or numerically controlled machine tools, sensors, among other things, must be included and monitored to insure safety. To guarantee correct operation, faults on the sensor connecting lines must be detected rapidly. If the connecting lines are protected in the control cabinet or in system parts, it can be assumed, as a rule, that a fault (short circuit, short between wires etc.) is highly improbable. A standard (prEN 954-2) allows a so-called fault exclusion to be assumed for the connecting line in this case.
Normally, sensors are connected to a safety control system mainly in accordance with either a 3-terminal concept or a 4-terminal concept. In the case of the assumption of a fault exclusion, a sensor design according to the 3-terminal concept is completely adequate. If this cannot be guaranteed throughout or if higher requirements are set, as in the case of an emergency-stop button, the sensor must be connected in accordance with the 4-terminal concept.
A concept for reliable fault detection of short circuits and cross-connections on the connecting lines must be appropriately designed. The present invention describes a method by which this can be guaranteed in a relatively simple and economic manner.
According to the known prior art, there are the abovementioned fundamental two possibilities of the 3-terminal concept and the 4-terminal concept for the safety-oriented actuation of a sensor. Both concepts will be explained in greater detail in the text which follows, with reference to the basic sketches shown in FIGS. 1 to 4.
The illustration of FIG. 1 shows a sensor connection according to the 3-terminal concept. In principle, all safety-oriented sensors have two decoupled signal-transmitting elements, shown as contacts K1 and K2 in a sensor Z (an emergency-stop button) in FIG. 1. These can be located in a single component (e.g. in the emergency-stop button) or in two physically separate components as is possible, for instance, in door switches for a protective door interlock.
The sensor Z is activated with a signal S1, which is conducted to the two contacts K1 and K2, via a common connection W from a test output A1, which can be provided, for example from a PLC (Programmable Logic Controller) with 24 V signal voltage. The two signals of the sensor Z are conducted to two input channels E1 and E2 of a safety-oriented control system (not shown). These signals are forwarded to a central control unit, such as via a system bus B, for further signal processing.
As already mentioned, safety components with contacts, e.g. emergency-stop buttons or standard components with contacts such as door switches for protective door interlocking, etc., can be used as a sensor if the two contacts are decoupled without mechanical interaction.
Faults in the connecting lines can be detected in conjunction with a cross-comparison of data and positive dynamization known to the expert. However, the 3-terminal concept cannot be used for detecting a pure cross-connection between the two inputs E1 and E2.
In the representation according to FIG. 2, a sensor connection employing the 4-terminal concept is shown. For this purpose, the sensor is activated by two test outputs A1 and A2 via separate signals S1 and S2, instead of via a common connection. The two signals of the sensor Z are connected to the two input channels E1 and E2 of the safety-oriented control system. For the rest, the illustration corresponds to that of FIG. 1.
A large proportion of the possible faults can already be detected with the 3-terminal concept in conjunction with the channel monitoring (cross-comparison of data) of the safety-oriented control system and a positive dynamization.
For example:
a line break in the lines, for instance, can be detected directly via a cross-comparison of data;
a short circuit or cross-connection to P potential (P stands for a positive or negative voltage) can be detected within the test cycle with the aid of positive dynamization;
a short circuit to M potential (M stands for ground potential) can be detected directly via a fuse;
a cross-connection to P potential can be detected within the test cycle with the aid of positive dynamization; and
a cross-connection to M potential can be detected directly via a fuse.
However, the 3-terminal arrangement does have short comings. For example, a cross-connection between the input channels E1 and E2 cannot be detected. A cross-connection between input channel E1 and E2 produces a so-called xe2x80x9csleeping faultxe2x80x9d in the system. In the illustration of FIG. 3, a cross-connection Q between the signal lines to E1 and E2 is additionally indicated in a development of the illustration of FIG. 2.
A second fault, for example a P short asxe2x80x94shown in FIG. 4 can also cause the safety function to fail. For a control category 3, this is still acceptable if the fault detection can be guaranteed within the test cycle established for this part of the safety-oriented functions (e.g., 8 hours).
Normally, for example, the emergency-stop button or the protective door interlocking switch is connected via an external safety switching device. In these devices, the monitoring of the connecting lines is integrated and includes cross-connection monitoring. There is a further possibility if the sensors are connected to a safety PLC. However, this requires special peripheral assemblies in which the short-circuit and cross-connection monitoring is also integrated. There is no solution at present which manages such faults with only standard assemblies.
The protection against short circuits and cross-connections is also conceivable with special arrangement and design of the connecting lines to the sensor. In such an arrangement, the lines must be individually shielded and the shield must be connected via a fuse in order to detect a short before it leads to failure of the protective function. In general, however, such a solution fails in practice due to a lack of acceptance.
Although the known external safety switching devices mentioned meet the requirements with respect to monitoring of the sensor connecting line, they can only switch off power and do not reliably stop a machine. This is also the reason why these devices are being replaced more and more by integrated safety concepts, especially in the case of industrial robots and machine tools. Thus, they do not represent a forward-looking alternative to the present invention.
It is true that, from a purely technical point of view, the problem can also be solved by using a safety PLC. A safety PLC can activate a sensor up to control category 4 (according to EN 954-1). The electronics in the input modules probe the line by so-called light/dark switching. In this process, the outputs are disconnected for a few milliseconds and a measurement is taken in this interval. In this way, short circuits and cross-connections are detected immediately and in parallel with the running process in a high-quality manner. However, this solution cannot be implemented by way of standard input/output modules. Thus, for cost reasons, the safety PLCs are not generally used today in machine tools or production machines. Today, this approach is only used in very large systems having very many ( greater than 40) safety-oriented signals and/or if a design in control category 4 is required at the same time.
Another conventional method for fault detection in safety-oriented signals is an alternating channel-related activation (cycling) of the sensor. Channel E1 and channel E2 are each alternately activated only once. However, this must not lead to the signal chain being switched off at any time. A check is made whether there is any crosstalk to the channel which is not activated. During this time, any cross-comparison of data implemented must be switched off. This results in increased risk. Further, although this makes it possible to achieve the objective, in principle, the expenditure in the peripherals is no less. However, the interactions with the cross-comparison of data are considerable.
Another conventional method for fault detection in safety-oriented signals is performed by way of different channel-related polarities of the voltage. In this solution, the conductor loops of the two contacts E1 and E2 are fed with voltages of different polarity, e.g. channel E1 with +24 V and channel E2 with xe2x88x9224 V. However, a special power supply must be provided for this purpose. Such a bipolar supply, however, is not provided as standard in many systems and this further increases the wiring complexity.
It is, therefore, the object of the present invention to create a method for fault detection in safety-oriented signals which is also capable of detecting so-called xe2x80x9csleeping faultsxe2x80x9d and which can be implemented with relatively inexpensive standard elements and components.
In particular, it is intended to achieve, in contrast to the 3-terminal concept, a cross-connection detection between the two channels E1 and E2 in conjunction with positive dynamization.
According to the present invention, this object is achieved by a method for fault detection in the operation of sensors with at least two decoupled signal-transmitting elements with contacts and with a closed conductor loop, in which method a respective signal is applied to the input of each signal-transmitting element via an associated test output, which signal is supplied to an associated input channel at the output end. Preferably, each test output is switched at a defined time delay compared with the other test outputs, in such a manner that the respective signals are present at the respective signal-transmitting elements with a corresponding switching delay and the respective input channels follow a defined sequence of signal changes with a fixed temporal correlation. Further, it is also preferable that the input channels are evaluated on the basis of these signal variations and the respective time interval corresponding to a defined expectation, with the presence of faults being inferred from deviations between actual and expected signal variations and/or time intervals.
In a first embodiment of the present invention, the object set forth above is achieved, primarily in the operation of sensors with a predominantly static signal, in that the method is performed during a positive dynamization.
According to a further embodiment of the present invention, the method is performed when each sensor is switched on.
In yet another embodiment of the present invention, the method is performed when each sensor is switched off.
To detect a cross-connection in the operation of sensors with two decoupled signal-transmitting elements with contacts and with a closed conductor loop, both test outputs being activated at a starting time, the following further method steps are performed in accordance with a further embodiment of the present invention
Deactivating a first test output at a first time; and
Generating a fault signal of the xe2x80x98cross-connectionxe2x80x99 category if the signal of the input channel associated with the first test output remains in the activated signal state.
To detect a pure cross-connection without phase short, the following further method steps are performed in accordance with yet a further embodiment of the present invention, namely, waiting for a defined switching delay, deactivating the second test output at a time after the switching delay has expired, and generating a fault signal of the xe2x80x98cross-connection without phase shortxe2x80x99 category if the signals of the two input channels together assume the deactivated signal state only after the switching delay for the second test output has expired.
To detect a cross-connection in the operation of sensors with two decoupled signal-transmitting elements with contacts and with a closed conductor loop, both test outputs being deactivated at a starting time, the method can also include the steps of activating a first test output at a first time, waiting for a defined switching delay, and generating a fault signal of the xe2x80x98cross-connectionxe2x80x99 category if the signals of the two input channels assume the activated signal state and also remain active after the switching delay has expired.
According to a further embodiment of the method described above, a cross-comparison of data is additionally performed between the signals of the two input channels for detecting a line break in a signal line.
The concept according to the invention presented can be applied when sensors with contacts and with closed conductor loops (closed-circuit principle) are used. In the case of electronic signals, the line monitoring by the sensor must be ensured. This is also the case with higher-quality sensors, such as light curtains and scanners. Using the method of the present invention, it is now possible to implement complete fault detection (including cross-connection) on the sensor connecting lines using standard modules.
The sensor is generally connected using the 4-terminal concept. As a result, no additional wiring is required and no special modules with integrated safety-oriented functions must be used. Apart from the time-delayed switching of the test outputs, the type of evaluation of the input channels is also significant. The evaluation performed with defined expectation is based on signal variation and/or time interval.
Further details and advantages of the present invention will become apparent from the description of preferred embodiments in conjunction with the figures, in which features having the same functionality are designated by the same reference symbols for the sake of better clarity.